This invention relates to refrigeration and, more particularly, to a cryogenic cooling system with cooldown and steady state or normal modes of operation for cooling a superconductive electric machine. As used herein, the term xe2x80x9ccryogenicxe2x80x9d is defined to describe a temperature generally colder than 150 Kelvin.
Superconducting devices include magnetic resonance imaging (MRI) systems for medical diagnosis, superconductive rotors for electric generators and motors, and magnetic levitation devices for train transportation. The superconductive coil assembly of the superconducting magnet for a superconductive device comprises one or more superconductive coils wound from superconductive wire and which may be generally surrounded by a thermal shield. The assembly is contained within a vacuum enclosure.
Some superconductive magnets are conductively cooled by a cryocooler coldhead (such as that of a conventional Gifford-McMahon cryocooler) which is mounted to the magnet. Mounting of the cryocooler coldhead to the magnet, however, creates difficulties including the detrimental effects of stray magnetic fields on the coldhead motor, vibration transmission from the coldhead to the magnet, and temperature gradients along the thermal connections between the coldhead and the magnet. Such conductive cooling is not generally suitable for cooling rotating magnets, such as may constitute a superconductive rotor.
Other superconductive magnets are cooled by liquid helium in direct contact with the magnet, with the liquid helium boiling off as gaseous helium during magnet cooling and with the gaseous helium typically escaping from the magnet to the atmosphere. Locating the containment for the liquid helium inside the vacuum enclosure of the magnet increases the size of the superconductive magnet system, which is undesirable in many applications.
What is needed are innovations in a cryogenic cooling system useful for cooling a superconductive device. Such cooling system must be remotely located from the magnet. Additionally, the cooling system should be capable of cooling a rotating superconductive magnet, such as that of an electric generator rotor.
One innovation directed to this need is disclosed in U.S. Pat. No. 5,513,498 to Ackermann et al. which is assigned to the intent assignee. This innovation employs a single compressor and a rotary valve for causing alternating circulation of a fluid cryogen, such as helium, in opposite directions in coolant circuits for cooling a superconductive device. While the innovation disclosed in the Ackermann et al. patent substantially overcomes the aforementioned problems, another innovation is still needed to meet the objectives of providing a cryogenic cooling system to cool down the rotor of a superconductive generator to an operating temperature and to maintain the rotor at that operating temperature for normal operation.
A cryogenic cooling system with cooldown and normal modes of operation is designed to achieve these two modes of operation with a forced flow helium cooling system that has both cooldown and normal modes of operation for cooling the superconductive coils of a rotating machine and for providing redundancy for improved system reliability.
In one embodiment of the invention, a cryogenic cooling system for a superconductive electric machine comprises means for defining a first circuit adapted to force flow of a cryogen to and from the superconductive electric machine and being operable in a cooldown mode for cooling the cryogen and thereby the superconductive electric machine to a normal operating temperature; and means for defining a second circuit adapted to force flow of a cryogen to and from the superconductive electric machine and being operable in a normal mode for maintaining the cryogen and thereby the superconductive electric machine at the normal operating temperature.